1. Field of the Invention
The present invention relates to a roller hemming device adapted to carry out a pre-hemming process and/or hemming process, using a robot manipulator to which a roller is attached
2. Description of the Related Art
In a roller hemming process wherein two metal sheets are joined so as to form a door panel or the like of an automobile, a peripheral edge of an outer sheet of the two metal sheets is vertically bent along the entire circumference thereof, the outer sheet is fixed to a mold, an inner sheet of the two metal sheets is stacked on the outer sheet, and then the two sheets are joined by pressing a roller attached to a robot manipulator against the peripheral edge of the outer sheet. In this regard, a process for further bending the vertically bent edge of the outer sheet by 45 degrees is referred to as a “pre-hemming process,” and a process for further bending the vertically bent edge of the outer sheet by 90 degrees (i.e., the peripheral edge becomes flattened) is referred to as a “hemming process.”
In order to obtain high processing quality, it is necessary to apply sufficient pressing force to the peripheral edge during the pre-hemming and/or hemming process. In the prior art, the pressing force is generated by a spring, hydraulic equipment or a servomotor, which is arranged in a tool at a wrist portion of a robot manipulator. For example, Japanese Unexamined Patent Publication (Kokai) No. 61-262432 discloses a roller hemming device having a robot hand and a hem roller attached to a front end of the robot hand, wherein the hem roller is rotationally driven by a motor arranged in the robot hand.
The pressing force of the hem roller or the pre-hem roller against a workpiece is an important factor for the pre-hemming or hemming process. For example, Japanese Unexamined Patent Publication (Kokai) No. 5-305357 discloses a roller hemming device having a hem roller adapted to move in a pressing direction, wherein the hem roller may press a workpiece to be hemmed by an appropriate pressing force along an accurate trajectory, even if a robot hand is deflected.
The processing quality or the shape of bent workpiece depends on the positional accuracy of the robot manipulator, since the roller is moved by the robot manipulator or the robot hand. In order to improve the positional accuracy, some techniques for reducing or eliminating positional error of the manipulator have been proposed. For example, Japanese Unexamined Patent Publication (Kokai) No. 7-60370 discloses a hemming device wherein a receiving roller and a pressing roller are arranged at a wrist section of a robot manipulator so that the positional relationship between the two rollers may be changed by means of a cylinder, in view of the deterioration of processing accuracy due to creep or displacement of a joint of the robot manipulator generated by a force applied to the manipulator during processing.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2003-103325 discloses a roller hemming device wherein a workpiece and a peripheral edge of a pedestal are nipped between hem rollers and driven rollers so as to carry out a hemming process.
Still furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2006-88217 discloses a roller hemming device having a guide rail positioned at the end of a peripheral flange of a pedestal, the guide rail being adapted to guide at least one of a hem roller or a pre-hem roller along a traveling direction of the process.
In the invention of Japanese Unexamined Patent Publication (Kokai) No. 61-262432, the hemming process may not be properly carried out due to a positional error on the robot manipulator relative to the workpiece. In the invention of Japanese Unexamined Patent Publication (Kokai) No. 5-305357, creep of displacement may occur at a bearing or the like of the joint of the robot, due to external force applied to the robot hand during the hemming process. In the invention of Japanese Unexamined Patent Publication (Kokai) No. 7-60370 or Japanese Unexamined Patent Publication (Kokai) No. 2003-103325, although the positional accuracy of the roller relative to the workpiece may be improved, many kinds of rollers are necessary, and thus the structure of the tool becomes complicated and cost thereof may be increased, and/or the reliability of the tool may be deteriorated. Furthermore, in the invention of Japanese Unexamined Patent Publication (Kokai) No. 2006-88217, it is necessary to arrange the guide rail for guiding the roller at the mold, and the mold is necessary to be prepared corresponding to each kind of workpiece. Therefore, it is troublesome to arrange the guide rail at the corresponding mold, which is disadvantageous in view of cost.
When the workpiece has a curved portion, high processing quality may be obtained by varying the pressing force depending on the location in the workpiece. However, it is difficult to vary the pressing force during roller hemming process.